SEC Chief's Exit Opens Void

When Mary Schapiro took over as chairman of the Securities and Exchange Commission in 2009, the agency was reeling from its failure to spot Bernard Madoff's yearslong fraud and the collapse of some of the investment banks it oversaw.

Ms. Schapiro, who said Monday that she would step down after four turbulent years, helped restore the agency's reputation and ensure its survival. But she also leaves behind a string of pressing regulatory issues—such as the so-called Volcker rule restricting proprietary trading at...